There are proposed a variety of kinds of recording media for recording or storing digital audio data, an image, a moving image, and a document file/data file created with use of a computer or a like device. An optical disc is known among the variety of kinds of recording media. A DVD (Digital Versatile Disk) has a high density and a large capacity, as compared with a conventional CD (Compact Disc), and has been spread as a recording medium for use in a recording device in place of a VTR (Video Tape Recorder). In recent years, development of a next-generation optical disc having a higher recording density, with use of a blue semiconductor laser, has been progressed in many research institutes, and rapid spread of the next-generation optical disc has been awaited.
It is necessary to increase the recording density of information by reducing the size of light spot to be defined by irradiation of light onto an optical disc in recording information in the optical disc and reproducing information recorded in the optical disc in order to increase the capacity of the optical disc. The size of light spot can be reduced by using a light source for emitting laser light of a short wavelength, and increasing the numerical aperture (NA) of an objective lens. In DVD, a light source having a wavelength of 660 nm, and an objective lens having a numerical aperture (NA) of 0.6 are used. For instance, use of a blue laser having a wavelength of 405 nm, and an objective lens having NA of 0.85 enables to obtain a recording density five times of the recording density of a currently available DVD. An optical information device is necessary to perform a data recording/reproducing operation with respect to the optical disc having the above specifications.
Providing compatibility with an existing optical disc in an optical information device for performing a high-density recording/reproducing operation with use of laser light of a short wavelength to be emitted from a blue laser is advantageous in enhancing the utility of the device, and increasing the cost performance of the device. In providing compatibility, it is difficult to secure a long operating distance, as in an objective lens for use in DVD or CD, while securing a large numerical aperture i.e. 0.85 for the objective lens. In view of this, in the compatible optical information device capable of performing a high-density recording/reproducing operation, it is desirable to individually provide at least one objective lens for use in recording and reproducing information on and from CD or DVD, and a high-density recording objective lens having a numerical aperture larger than the numerical aperture of the one objective lens.
An optical pickup device of a first conventional art has multiple objective lenses in a movable section to realize a compatible optical information device capable of recording and reproducing on and from multiple optical discs different from each other in recording density (see e.g. patent document 1). FIG. 19 is a diagram showing an arrangement of the optical pickup device of the first conventional art.
A first light beam emitted from a first light source 61 is converted into substantially parallel light by a collimator lens 62. Then, an optical axis of the first light beam is bent by a half mirror 63, and is further bent by a rise-up mirror 64 in a direction orthogonal to a first optical disc (not shown) having a high recording density. A first objective lens 65 is operable to converge the first light beam on a recording surface of the first optical disc. Similarly to the first light beam, a second light beam emitted from a second light source 71 is converted into substantially parallel light by a collimator lens 72. Then, an optical axis of the second light beam is bent by a beam splitter 73, and is further bent by the rise-up mirror 64 in a direction orthogonal to a second optical disc (not shown) having a recording density lower than the recording density of the first optical disc. A second objective lens 75 is operable to converge the second light beam on a recording surface of the second optical disc.
As described above, the optical pickup device of the first conventional art includes: at least two objective lenses i.e. the first objective lens 65 for condensing the first light beam to be emitted from the first light source 61 on the information recording surface of the first optical disc, and the second objective lens 75 for condensing the second light beam to be emitted from the second light source 71 on the information recording surface of the second optical disc; and the rise-up mirror having at least two reflection surfaces i.e. a reflection surface for reflecting the first light beam for incidence into the first objective lens 65, and a reflection surface for reflecting the second light beam for incidence into the second objective lens 75. This arrangement enables to realize a compatible optical pickup device capable of recording and reproducing on and from multiple optical discs different from each other in recording density.
An optical pickup device incorporated with a blue semiconductor laser of a second conventional art includes an optical component made of a resin, or an optical element formed by cementing light transmissive members to each other by a UV curable adhesive or a like agent. In use of the optical pickup device having the above arrangement, irradiation of blue laser light may lower the transmittance of the optical component, or affect the composition of the light transmissive member, thereby damaging the optical element. In view of this, various measures have been proposed to enhance the light resistance of the light transmissive member against blue laser light (see e.g. patent documents 2 and 3).
An optical pickup device of a third conventional art has light blocking means for blocking laser light (see e.g. patent document 4). FIGS. 20A and 20B are diagrams showing an arrangement of the optical pickup device of the third conventional art. Referring to FIGS. 20A and 20B, a light blocking plate 80 is provided to prevent stray light in a first optical unit 81 and a second optical unit 82 from being incident into the counterpart unit as noise to thereby prevent mutual interference of laser light.
In the first conventional art, multiple light sources are provided in a single optical pickup device to realize a compatible optical pickup device capable of recording and reproducing on and from multiple optical discs different from each other in recording density. In the case where a blue semiconductor laser is used as one of the light sources, it is necessary to use an optical element having high light resistance against blue light, as recited in the second conventional art, as all the optical elements through which a light beam to be emitted from the blue semiconductor laser is supposed to be transmitted. The optical pickup device of the first conventional art has an optical arrangement that an optical system having a blue semiconductor laser as a light source, and an optical system having a semiconductor laser for emitting light in a red wavelength band and an infrared wavelength band as a light source, are disposed opposite to the rise-up mirror 64 having at least two reflection surfaces so that the individual light fluxes are incident into the rise-up mirror 64.
FIGS. 21A and 21B are side views of a rise-up mirror of a conventional optical pickup device. In this arrangement, as shown in e.g. FIG. 21A, a blue light beam (i.e. leaked light 67) which has been partially transmitted through a reflection surface 64a of the rise-up mirror 64 is transmitted through the other reflection surface 64b after having been reflected on an inner surface of the rise-up mirror 64 or a like operation. Thereafter, the blue light beam is incident into an optical system for guiding red/infrared light. Further, as shown in e.g. FIG. 21B, upon irradiation of a blue light beam in the vicinity of a vertex 64t of the rise-up mirror 64, the blue light beam is scattered, and scattered light 68 is incident into the objective lens 75 for guiding red/infrared light.
Because of the above phenomenon, use of a resin component, as an optical component for a semiconductor laser for emitting laser light in a red wavelength band or an infrared wavelength band, may cause damage of an optical element, transmittance lowering, or wavefront aberration degradation, with the result that the recording/reproducing performance of the optical pickup device may be degraded, because generally the optical element does not have light resistance against blue light. It is possible to reduce the light amount of a blue light beam passing through the optical component for guiding red/infrared light by limiting the emission light amount of the blue light beam in order to suppress damage or performance degradation of the optical element. However, if the above measure is taken, it may be difficult or impossible to secure a light amount of a blue light beam sufficient for performing a recording/reproducing operation with respect to a multi-layered disc, or a high-speed recording/reproducing operation. This may lower the recording/reproducing quality of the optical pickup device.
The third conventional art discloses an arrangement, considering noise which may be generated by mutual interference of laser light. The light blocking plate 80 is provided solely between two rise-up mirrors, and the height of the light blocking plate 80 is not larger than the height of the rise-up mirrors. Stray light may be generated in the optical pickup device in a region other than the region between the rise-up mirrors. For instance, there is a likelihood that scattered light from one side of the rise-up mirror may be incident into a component on an optical path defined by bending the optical axis of laser light in a direction substantially perpendicular to a bottom surface of the optical pickup device by the rise-up mirror, as stray light from the counterpart optical system (optical unit).
The light blocking plate 80 in the third conventional art is not a light blocking plate for blocking light of a specific wavelength. The third conventional art does not consider a case that light of a short wavelength e.g. blue violet light, other than the wavelength of red light or infrared light, may be incident. In the arrangement, wherein the light blocking plate 80 may allow transmitting of light of a short wavelength e.g. blue violet light, there is a likelihood that one of the optical components on the optical path may be exposed to blue violet light, thereby causing wavefront aberration degradation or transmittance lowering.
Patent document 1: JP 2005-293686A
Patent document 2: JP 2005-141016A
Patent document 3: JP 2005-306896A
Patent document 4: JP 2002-208173A